1. Field of the Invention
This invention relates in general to motor speed control systems. More specifically, the invention provides a control system which is capable of improving a disturbance response characteristic of such a control system.
2. Description of the Prior Art
A control system generally includes a manipulated mechanism that operates a controlled device in accordance with a command value of a manipulated variable, a detector that detects a control variable of the controlled system, and a control processing unit that receives a detected value of the control variable which is the output of the detector and the command value of the control variable so as to produce the command value of the manipulated variable; there has been provided, for example, a motor speed control system using a power converter.
Hereafter, this invention will be described, taking as an example a motor speed control system.
In a motor speed control system, there is generally provided a speed detector coupled to a motor, and the detected speed value derived from the speed detector is compared with a speed command value so that the current of the power converter is controlled, whereby a feedback control is performed.
FIG. 1 (Prior Art) shows a fundamental configuration of a speed control system that controls the speed of a DC motor in a feedback control fashion.
Reference numeral 1 designates a speed control amplifier, 2 a power converter, 3 and 4 armature and field windings, respectively, of a DC motor, 5 a speed detector, and 6 a load of the motor, respectively. A field exciting circuit for field winding 4 of the DC motor is not shown. Power converter 2 includes controlled rectifiers, choppers and a current control circuit that controls an output current I to armature winding 3.
It is assumed that power converter 2 can be regarded as a current amplifier that provides its output current I in accordance with a current command value such as, for example, a torque command value signal T*.
Speed control amplifier 1 receives a speed command value signal n* provided by a reference value setting unit 9 and a detected speed value signal n from speed detector 5. In response to these two input signals, speed control amplifier 1 provides torque command value signal T* corresponding to a torque value required for the motor. In the case of a DC motor, its torque and armature current are proportional to one another, so that the torque command value signal T* is inputted to power converter 2 as the current command value signal.
In response to signal T*, power converter 2 causes current I to flow in armature winding 3 of the motor. In response, the motor produces a torque proportional to the torque command value signal T*, whereby the speed thereof is varied. Speed n is detected by speed detector 5 and fed back to speed control amplifer 1.
The predominant desirable characteristics of a speed control system are such that (a) motor speed can respond rapidly and accurately with respect to changes in the speed command value, (b) motor speed change due to load variation can be kept to a minimum and motor speed can be rapidly restored after a load variation. Characteristic (a) is known as a command value response characteristic, and characteristic (b) is known as disturbance response characteristic. The predominant system component determining such characteristics is speed control amplifier 1.
A conventional speed control amplifer employs a so-called control amplifier such as a proportional control amplifier, a proportional integral control amplifier, or a proportional integral control amplifier with compensation for anti-overshoot.
In addition to these, various control amplifiers are used depending on the respective purposes to which the apparatus is to be applied. These control amplifiers are designed such that the command value response is regarded as being of major importance, and the disturbance response is, in general, slower than the command value response.
FIG. 2 is a view illustrating one example of the command value response and the disturbance response in the case where the proportional integral control amplifier with anti-overshoot compensation is used as a speed control amplifier. In FIG. 2, the abscissa represents time (seconds) and the ordinate a speed n, respectively, and at a time of 0.0 second a speed command is changed from 0 to 1, and at a time of 0.5 second a certain constant load is applied and the response thereat is shown. As shown, the command value response time is approximately 0.1 second while the disturbance response times is approximately 0.3 second, and the ratio of these response time is approximately 3.
The motor speed control system is used, for example, to drive a rolling mill that rolls steel, and in this application, when the steel passes through the rolling mill, there is applied an abruptly drastic load, so that if its disturbance response characteristics were not superior, it would be difficult to ensure uniformity in thickness of the rolled steel, thus superiority of the disturbance response is regarded as being of major importance rather than command value response.